Complexes of cobalt(iii) with phenolate-containing polydentate ligands and bovine serum apo-transferrin: towards creating spectroscopic models for cobalt(iii)–tyrosinate interactions

Abstract

Reaction of each member of a series of variously substituted N-(2-hydroxybenzyl)glycinate (hbg²⁻) ligands [5-bromo- (5-Br-hbg²⁻); 5-chloro- (5-Cl-hbg²⁻); 3,5-dichloro- (3,5-Cl₂-hbg²⁻); 5-chloro-3-methyl- (5,3-Cl,Me-hbg²⁻); and 3,5-dimethyl- (3,5-Me₂-hbg²⁻)] with cobalt(II) in stoichiometric amounts under ambient conditions afforded the Co^III complex anions of the general formula trans,trans,trans-[Co(hbg)₂]⁻. Single-crystal X-ray analyses of [C₅H₁₀NH₂]-trans,trans,trans-[Co(3,5-Cl₂-hbg)₂] 3 and [C₅H₁₀NH₂]-trans,trans,trans-[Co(3,5-Me₂-hbg)₂] 5 {[C₅H₁₀NH₂] = piperidinium counter ion} have revealed similar structural motifs of these compounds. Each of the crystal structures shows two non-equivalent complex anions in the unit cell connected through hydrogen bonding via the piperidinium cation. The two hbg²⁻ ligands in each complex ion are arranged facially around the cobalt(III) ion and each ligand employs a phenolate oxygen, an amine nitrogen and a carboxylate oxygen to form a distorted octahedral geometry with C_i symmetry. These compounds (1–5) serve as spectroscopic models for the interaction of cobalt(III) with tyrosinate ligands. The electronic spectrum of the compound [C₅H₁₀NH₂]-trans,trans,trans-[Co(5,3-Cl,Me-hbg)₂] 4 bears a striking resemblance to that of the complex of cobalt(III) with bovine serum transferrin, [Co₂BTf]. In contrast to previous studies on the spectroscopy of the transferrins, one of the two ligand-field transitions expected to occur in [Co₂BTf] is evident in the electronic spectrum of this derivative of bovine serum transferrin in vitro. One of the fascinating characteristics of the trans,trans,trans-[Co(hbg)₂]⁻ model system is the solvent dependence of the optical spectra.